In technical applications such as, for example, motor vehicle construction and the like, it is often necessary to quickly and reliably detect the location of a component which can be moved into two end positions relative to a stationary part using measurement engineering. In the case of a belt lock of a safety belt system, for example, in a motor vehicle, it has to be checked whether a passenger is belted or not. Knowledge of the locking state of the belt lock is necessary to notify the passengers by a signal to put on and lock the safety belts. Since the introduction of the safety airbag, information about the locking state of the safety belts has also been important for activation or deactivation of mechanisms for inflating driver and passenger airbags as well as side and head airbags.
Known techniques disclose, for example, Hall sensors for contactless monitoring of components which change their position, especially which can assume two different end positions. Hall sensors consist in principle of a semiconductor layer which is supplied with constant current, generally in an integrated construction. The constant current is influenced by a magnetic field component perpendicular to the semiconductor layer, and the sensor delivers a Hall voltage which can be evaluated, which can be tapped and which can be used for evaluating a state and also directly as switching voltage. The integrated construction of Hall sensors allows for the integration of an evaluation circuit which is suitable for evaluating the operating state on the Hall sensor. Therefore, in the automobile industry, Hall sensors are used as contactless state sensors in many applications.
EP-A-0 861 763 discloses a belt lock with an integrated, biased Hall sensor which detects, without contact, the state of a locking body or an ejector for a lock tongue which has been inserted into a belt lock. Here, a Hall sensor with a Hall field is located in direct proximity to a permanent magnet. By changing the location of the locking body or of the ejector which for this purpose consists of a ferromagnetic material, the magnetic field of the permanent magnet is changed. In doing so, the signal of the Hall sensor changes and at the output of the Hall sensor the state change can be tapped as a voltage change. In one alternative version, it is suggested that the Hall sensor with a Hall field be installed without a permanent magnet and for this purpose the locking body or the ejector be made as permanent magnets. In this arrangement, the change of the position of the locking body or of the ejector should also be detectable by a change of the Hall voltage.
A drawback in the belt lock according to EP-A-0 861 763 is that the Hall sensor must be positioned very carefully with respect to the locking element or the ejector. Subsequent installation of the Hall sensor is therefore relatively complex and expensive. Depending on its arrangement, the Hall sensor is also sensitive to stray external magnetic fields which can be caused, for example, by a magnetic key chain. Optionally, even additional shielding must be mounted, which makes mounting or installation even more complicated. The susceptibility to stray external fields is also increased by the signal changes being relatively small due to the comparatively short distances which are traversed by the locking body or the ejector when closing or opening the safety belt lock. The belt lock version without a biased Hall sensor, in which either the locking body or the ejector are made as a permanent magnet, is less practicable. The attainable signal changes are also relatively small here; this makes detection of different states difficult, whether the belt lock is open or closed. Over time, the permanent magnet can be demagnetized due to vibrations of the locking body and of the ejector when the safety belt is open or closed. This can ultimately lead to the Hall sensor becoming ineffective and the state changes of the belt lock no longer being able to be reliably detected.
The known belt locks all have a very compact construction. The space available within the belt lock is therefore generally very limited. This makes it difficult to arrange the sensor components within the belt lock housing, especially in the vicinity of a component which, when the belt lock is activated, changes its position from one end position into the other end position. If shields are also then to be mounted, the designer is generally faced with an essentially insoluble problem since the dimensions of the belt lock housing are not to be changed.
EP-1 485 276 B1 discloses a mechanical switch which can be used as a sensor for monitoring the locking state of a belt lock. The switch has two contact sheets which are arranged in a switch housing. The contact-making regions of the contact sheets are located in any upper housing interior of the switch housing. One of the contact sheets is made as a contact spring with a hammer-shaped region on the contact-making end which interacts with a fork-shaped contact-making end of the fixed contact sheet. The contact spring has a middle, arc-shaped region which projects into a channel which is provided laterally from the housing interior. The contact of the two contact sheets is closed or opened by moving a slide in the channel.
This known mechanical switch is insensitive to stray electromagnetic fields. It does not have any permanent magnets either which could become demagnetized as a result of the vibration over time and thus which could adversely affect the operation of the sensor. The known mechanical switch is one which is made comparatively compact and can be housed in various belt lock versions without the need for major modifications on the belt lock housing or on the belt lock itself. A drawback in the known mechanical switch is the relatively complex shape of the contact sheets which requires a very accurate calibration to one another. The middle arc-shaped region of the contact sheet which is made as contact spring must be made very exactly so that the operation of the switch is ensured. The hammer-shaped contact region adjoining it requires relatively high precision so that contact with the fork-shaped contact regions of the stationary contact sheet is ensured. Providing an upper housing interior for the contact-making regions of the contact sheets which is designed to prevent any penetration of foreign bodies dictates a switch housing of complex shape. In order to achieve the required precision, a relatively high production effort must be exerted which has adverse effects on the costs for the switch.